I. Field of the Invention
The present invention relates generally to satellite communication systems, and more particularly, to a method and apparatus for measuring and correcting the error in a user terminal clock.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters, and U.S. patent application Ser. No. 08/368,570, entitled Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy, both of which are assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which a large number of generally mobile or remote system users or subscriber units ("user terminals") employ at least one transceiver to communicate with other user terminals, or users of other connected systems, such as a public telephone switching network. Communication signals are transferred either through satellite repeaters and gateways, or directly to terrestrial base stations (also sometimes referred to as cell-sites or cells).
In a modern satellite communications system, timing is critical. For example, such systems typically divide communications channels into "frames" where each frame is of a known duration. In order to optimize the use of such frames, the gateways or base stations and the user terminals must employ some method to ensure synchronization. Therefore, each user terminal is supplied with a device for providing a timing reference. An ideal time reference would supply the user terminal with a signal of a known frequency.
A local oscillator is often used to provide a timing reference in the user terminal. However, no local oscillator is perfect. Local oscillators are subject to frequency drift. When the frequency of the local oscillator drifts, synchronization is lost.
One approach to minimizing local oscillator frequency drift is to fabricate a more accurate local oscillator. However, such very stable local oscillators are very expensive to fabricate.
Another approach, commonly used in cellular telephone systems, involves the use of a voltage controlled temperature compensated crystal oscillator (VTCXO). The VTCXO is highly resistant to frequency drift caused by temperature changes. In addition, the output frequency of a VTCXO can be controlled by varying an input voltage to the VTCXO.
In such a cellular telephone system, each user terminal is supplied with a VTCXO. Each user terminal monitors a pilot signal transmitted by a base station. The user terminal uses the frequency of the pilot signal as a timing reference to adjust the output frequency of the VTCXO by varying the input voltage applied to it. Such an approach can be used in a cellular telephone system because the relative radial velocities between the base stations and the user terminals are small. However, in some satellite communication systems, such as low-earth orbit (LEO) satellite communication systems, the relative radial velocities between a satellite and a user terminal can be very large. This large relative radial velocity imposes a large Doppler shift on the pilot signal, rendering it unusable as a timing reference.
The Doppler shifts are particularly acute in a LEO satellite system. For example, for a LEO satellite with a velocity of 7 kilometers/second, the Doppler shift seen by a user terminal is approximately 20 parts/million (ppm) at the horizon. For a LEO satellite transmitting at S-band (approximately 2.5 gigahertz) a Doppler shift of 2 ppm translates to a 50 kilohertz frequency shift. Such frequency shifts would render the pilot signal unusable as a timing reference.